Heat pump water heater and method of heat exchange



Patented July 18, 1 950 HEAT PUMP WATER HEATER AND METHOD OF HEAT EXCHANGE Alonzo W. Rulf, York, Pa., assignor to V. 0. Patterson & Associates, Inc., York, Pa., a corporation of Pennsylvania Application May 5, 1949, Serial No. 91,595

7 Claims. (Cl. 62-115) My invention relates to heat pumps and methods of heat pump operation. More particularly, this invention relates to heat pump water heaters and method of operating the same.

The broad concept of heat pumps is well known and there are installationsof equipment of this type which absorbheat from the ground or from underground water and transfer this heat to the interior of a house. It is also known to apply this principle to domestic hot water heaters. A specific example-of the latter is to be found in the U. S. patent to Wilkes 2,095,017 of 1937. Although it is well known that the efficiency of apparatus of this type may be greater than 100%, the problem of the art has been to maintain an efliciency over a long period of time which is sufficiently greater than 100% to make the apparatus competitive with gas, oil, and electrical heating This is so because electricity is still a relatively expensive source of energy for heating purposes throughout the United States. The expense of electrical power, plus the fact that when the demand is high, for example in the winter season, the efficiency is relatively low, are thefactors which have hampered commercial success of apparatus of this type.

Accordingly it is an object of my invention to provide a hot water heater operating on the heat pump principle which will have a suificiently high year-around coefficient of performance to enable it to compete with other forms of heating.

My invention will best be understood from the following description read with reference to the accompanying drawings in which,

Fig. 1 shows my invention in diagrammatic form,

Fig. 2 is a pressure-heat chart which shows the heat cycle for the apparatus of my invention.

As shown in Fig. 1, the basic system comprises a refrigerant evaporator unit I 0, a hot water storage tank H, a motor-compressor unit I2, and a refrigerant conduit comprising three series-connected heat transfer coils l3, l4, and I located respectively near the top, the middle, and the bottom of the-tank. The high pressure side of the compressor delivers gaseous refrigerant to coil H. A liquid refrigerant receiver 16 is connected between coils l4 and I5 and a thermal expansion valve H is connected between coil and the evaporator unit ID. A connection from the outlet of the evaporator to the low pressure side of the compressor completes the refrigerant circuit. The line It is a capillary equalizer line which may be connected between the high and low pressure sides of the system, in order to reduce the starting load on the motor-compressor. The fan I 9 insures 'a flow of air through the evaporator coil structure to provide a source of heat.

The operation of my apparatus is readily understood from the chart of Fig. 2 and the diagrammatic showing of the apparatus in Fig. 1.

Refrigerant in coils l3, I4, and 15, which has been placed under pressure by the compressor unit, is allowed to expand into the evaporator coils where it absorbs heat from the surrounding atmosphere. The gas thus heated is then pumped out of the evaporator through the compressor and into the coils l3, it, and B5. The heat due to the compression of the refrigerant, plus the heat picked up from the surrounding atmosphere is then transferred from the refrigerant to the water in the tank through the intermediary of heat transfer means such e. g., as coils l3, l0, and it.

Fig. 2 shows the heat cycle of my invention when using Freon-12 as the refrigerant. Since I do not limit myself to the use of this particular refrigerant, it is well to point out that the temperatures and pressures indicated in this figure are measured test data taken while using Freon 12, and are not necessarily those which would result under conditions of actual use with other refrigerants. The chart nevertheless serves to show the principles involved.

Starting at the lower right hand corner which corresponds to the low pressure side of the compressor, refrigerant in gaseous form at a temperature of about F. is compressed to a pressure of about pounds per square inch. At this pressure the gas has acquired a temperature of about F. corresponding to a superheat of about 60 F. Passing the gas in this condition through the uppermost coil it in the tank serves to cool the gas down to a saturated vapor state, as indicated by the intersection of the cycle curve with the saturated vapor line on the chart. From there, the saturated vapor at about 120 F. passes into the center coil It of the tank where further transfer of heat to the surrounding water results in condensing the gas. Since substantially all of the heat absorbed by the water from this coil is latent heat of condensation, there is little or no change in the temperature of the refrigerant. The refrigerant leaving the condensing coil is largely liquid but is passed first to the liquid receiver it which serves as a liquid seal and insures that only liquid refrigerant passes to the coil l5. This point on the chart is at the upper left hand corner where the cycle intersects the liquid line. The coil 15 through which the liquid refrigerant is then passed, is located in a stratum of water which is only slightl above the temperature of the supply, so that the refrigerant is further cooled by transfer of heat to this water. Since the refrigerant is liquid in this coil, the cooling takes place along the liquid line to the lower left hand corner of the diagram. From this point the liquid passes to the expansion valve I1 and vaporizes in the coils of the evaporator unit 10. This expansion or evaporation is shown by the substantially horizontal line at the bottom of the diagram which represents the absorption of heat from the surrounding atmosphere and completes the heat cycle. Referring again to Figure 1 of the drawing it should be noted that the level of liquid in the heat exchanger is located between the coils l4 and IS in the liquid receiver I6. It is of course necessary to maintain the charge of refrigerant in the system, to a point which will sustain this liquid level under operating conditions, otherwise the gain in efllciency due to liquid subcooling will be lost.

The above description applies to the operation of the apparatus of my invention after the unit has been in operation long enough to raise the water temperature to a degree approaching actual operating conditions. When the apparatus is started up cold, there will be little if any superheat in the refrigerant, and the initial heating of the Water is due principally to latent heat of condensation in both coils l3 and it. As the water in the tank warms up, however, it stratifles to a marked degree with the hottest water, at about 140 F. surrounding the coil i3 in the upper part of the tank. Just below this layer there will be a second stratum of water at about 120 F. surrounding coil i4, while the water in the bottom of the tank surrounding coil l5 will be about 70 F. if the temperature of the make-up water is about 65 F.

The vertical dotted line which cuts across the left hand portion of the heat cycle chart of Fig. 2 serves to illustrate the gain in heat transfer which results from subcooling the refrigerant in liquid form. This has proved to amount to an increase of about 20% in the efficiency over the old cycles in which the condenser coil was connected directly to the evaporator unit through the expansion valve.

The foregoing will enable one skilled in the art to fully understand my invention, and the novelty thereof which is defined by the following claims:

I.claim: c

1. A hot water heater of the heat pump type, comprising in combination: a hot water storage tank; a refrigerant evaporator adapted to absorb heat from the atmosphere; heat transfer means of the type in which refrigerant is circulated, said means being disposed in heat exchange relation to the water in said tank, extending substantially from top to bottom thereof and including, during normal operating conditions, a desuperheating stage located adjacent the upper part of said tank, a condensing stage located substantially at the midsection of said tank, and a subcooling stage located adjacent the bottom part of said tank in which the refrigerant is maintained in liquid phase; means connecting said subcooling stage to said evaporator, said means including an expansion device; a motorcom'pressor unit connected to pump refrigerant from said evaporator at low pressure and deliver said refrigerant at high pressure to the desuperheating stage of said heat transfer means; and means responsive to the temperature of the water in said tank to control the transfer of heat thereto.

2. A hot water heater of the heat pump type, comprising in combination: a hot water storage tank; a refrigerant evaporator adapted to absorb heat from the atmosphere; a closed heat transfer conduit for the refrigerant located in heat exchange relation to the water in said tank and extending substantially from top to bottom thereof, said conduit including a desuperheating part adjacent the upper section of said tank, a condensing part substantially at the midsection of said tank, and a subcooling part adjacent the bottom section of said tank in which the refrigerant is maintained in liquid phase; means connecting said subcooling part of said conduit to said evaporator, said means including an expansion device; a motor-compressor unit connected to pump refrigerant from said evaporator at low pressure and deliver said refrigerant at high pressure to the desuperheating part of said heat transfer conduit means; and means responsive to the temperature of the water in said tank to control the transfer of heat thereto.

3. A hot water heater of the heat pump type, comprising in combination: a hot water storage tank; evaporating means for the refrigerant to absorb heat from the surrounding atmosphere;

a first coil serving as a refrigerant desuperheater and located in heat exchange relation to the water in the upper part of said tank; a second coil serving as a refrigerant condenser, connected to said first coil and located in heat exchange relation to the water in the midsection of said tank; a liquid receiver for the refrigerant having an inlet and an outlet, said inlet bein connected to said second coil; a third coil serving as a subcooler for the liquefied refrigerant and located in heat exchange relation to the water in the bottom part of said tank, said third coil being connected to the outlet of said liquid receiver; means connecting the lower end of said third coil to said evaporator said means including an expansion device; a motor-compressor unit located in heat exchange relation to the water in said tank and connected to pump refrigerant at low pressure from said evaporator unit and supply it at high pressure to said first coil; and means responsive to the temperature of the water in said tank adapted to control the transfer of heat to the water.

4. A hot water heater as defined by claim 1 and including a capillary equalizer line connected between the bottom of said condensing stage and said evaporator.

5. A method of heating water in a storage tank with heat pump apparatus of the type in which a refrigerant is used as the working fluid and in which an evaporator is used to absorb heat from the atmosphere and a heat exchanger to transfer that heat to the water in the tank, the steps comprising: expanding a refrigerant into said evaporator to absorb heat from the surrounding atmosphere; raising the pressure of said refrigerant to place it in a superheated condition; desuperheating said refrigerant in heat exchange relation to the water in the upper part of said tank; condensing said refrigerant in heat exchange relation to the water in the midsection of said tank; and subcooling said condensed refrigerant in heat exchange relation to the water adjacent the bottom of said tank, while maintaining at all times, the water in the bottom part of the tank at a temperature below the temperatureof the water in the midsection of the tank.

6. A method of heating water in a storage tank with heat pump apparatus of the type in which a refrigerant is used as the working fluid and in which an evaporator is used to absorb heat from the atmosphere and a heat exchanger to transfer that heat to water in the tank, the steps comprising: initially charging the heat pump apparatus with a suflicient quantity of refrigerant to maintain during operating conditions a substantially liquid column in the lower portion of the heat exchanger; expanding said refrigerant into said evaporator to absorb heat from the surrounding atmosphere; raising the pressure of said refrigerant to place it in a superheated condition; desuperheating said refrigerant in heat exchange relation to the water in the upper section of said tank; condensing said refrigerant in heat exchange relation to the water in the midsection of said tank; and subcooling said condensed refrigerant in heat exchange relation to the water adjacent the bottom of said tank, while maintaining, at all times, the water in the bottom part of the tank at a temperature below the temperature of the water in the midsection of the tank.

7. A hot water heater of the heat pump type, comprising in combination: a hot water storage tank; a refrigerant evaporator adapted to absorb heat from the atmosphere; heat transfer means of the type in which refrigerant is,

circulated, said means being disposed in heat exchange relation to the water in said tank, extending substantially from top to bottom there- 'of and including, during normal operating conditions, a desuperheating stage located adjacent the upper part of said tank, a condensing stage located substantially at the midsection of said tank, and a subcooling stage located adjacent 6 the bottom part of said tank in which the refrigerant is maintained in liquid phase; means connecting said subcooling stage to said evaporator, said means including an expansion device; a motor-compressor unit connected to pump refrigerant from said evaporator at low pressure and deliver said refrigerant at high pressure to the desuperheating stage of said heat transfer means; and an electric switch to control the transfer of heat to the water in said tank, whereby the water in the bottom part of the tank and surrounding said subcooling stage is maintained at a temperature below the temperature of the refrigerant in said condensing stage.

ALONZO W. RUFF.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,064,272 Wolf June 10, 1913 1,320,216 Fisher Oct. 28, 1919 1,874,803 Reed Aug. 30, 1932 1,937,288 McGraw Nov. 28, 1933 2,042,812 Tull June 2, 1936 2,095,017 Wilkes Oct. 5, 1937 2,162,245, Momstock June 13, 1939 2,258,458 Lange Oct. 7, 1941 2,375,157 Wilkes May 1, 1945 2,493,141 Henney Jan. 3, 1950 FOREIGN PATENTS Number Country Date 4,336 Australia 1931 Certificate of Correction Patent No. 2,516,093 July 18, 1950 ALONZO W. RUFF It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 5, line 8, for the word substantially read substantial;

and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the case in the Patent Oflice.

Signed and sealed this 26th day of December, A. D. 1950.

THOMAS F. MURPHY,

Assistant C'Ommz'ssz'onerof Patents. 

